First problem: it doesn't tell me what the problem is, only some guesses at what the problem might be.
$ sudo do-release-upgrade
Calculating the changes
Could not calculate the upgrade
An unresolvable problem occurred while calculating the upgrade.
This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu
Second problem: None of these guesses are correct. I found out via the forums (not by any message from the software nor by reading `man do-release-upgrade` or any files in /usr/share/doc/ubuntu-release-upgrader-core/* )(which I might call a problem first-and-a-half) that a log file is written to /var/log/dist-upgrade/main.log
Third (actual) problem: do-release-upgrade wants to remove 'unity' and wants not to remove 'unity' at the same time, halting the upgrade.
$ grep -C3 ' ERROR ' /var/log/dist-upgrade/main.log
2013-10-18 09:26:15,596 DEBUG Marking 'xubuntu-desktop' for upgrade
2013-10-18 09:26:15,702 DEBUG blacklist expr 'unity$' matches 'unity'
2013-10-18 09:26:15,702 DEBUG The package 'unity' is marked for removal but it's in the removal blacklist
2013-10-18 09:26:15,789 ERROR Dist-upgrade failed: 'The package 'unity' is marked for removal but it is in the removal blacklist.'
2013-10-18 09:26:15,790 DEBUG abort called
2013-10-18 09:26:15,791 DEBUG openCache()
2013-10-18 09:26:15,791 DEBUG failed to SystemUnLock() (E:Not locked)
How do I resolve this?
I'm pretty sure I didn't manually forbid unity from being installed:
... I don't see a 'hold' on unity packages.
$ dpkg --get-selections |grep unity
(and while I'm looking at the preview, why aren't \[code\] blocks using a monospace font for whitespace characters?)